In general, in the case where a silicon single crystal substrate is etched with a chemical solution, there is carried out a method of performing etching with an acid based etching liquid which is a mixed aqueous solution having components such as hydrofluoric acid and nitric acid, etc. added thereto; or a method of performing etching with an alkali based etching liquid which is an aqueous solution of potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), etc. (see Non-Patent Documents 1 and 2).
In the case where an acid based etching liquid is used, the surface of silicon is oxidized with a component having an oxidizing action, such as nitric acid, etc., to form silicon oxide, and this silicon oxide is dissolved as silicon fluoride by hydrofluoric acid, etc., whereby etching proceeds. A characteristic feature in performing etching with an acid based etching liquid resides in the fact that even when silicon which is an etching object is monocrystalline, polycrystalline or amorphous, the etching isotropically proceeds. For that reason, in performing pattern etching using a pattern mask, etc., there may be the case where the deeper the etching, the more the etching in a lateral direction, namely undercut (erosion) beneath the pattern mask proceeds to the same degree as the depth, resulting in causing inconvenience.
On the other hand, in the case where an alkali based etching liquid is used, silicon is dissolved as a silicate ion by a hydroxy anion in the liquid, and on that occasion, water is reduced to generate hydrogen. When etching with the alkali based etching liquid is performed, different from the case of the acid based etching liquid, etching of monocrystalline silicon proceeds while keeping anisotropy. This is based on the fact that there is a difference in a dissolution rate of silicon in every crystal face orientation of silicon, and this etching is also called crystal anisotropic etching. Even in polycrystalline silicon, etching proceeds while keeping anisotropy on microscopic observations. However, in view of the fact that the face orientation of crystal grains is randomly distributed, isotropic etching appears to proceed on macroscopic observations. In amorphous silicon, etching isotropically proceeds on both of microscopic observations and macroscopic observations.
As the alkali based etching liquid, besides the aqueous solution of KOH or TMAH, an aqueous solution of sodium hydroxide (NaOH), ammonia, hydrazine, etc. is used. In etching processing of a monocrystalline silicon substrate using such an aqueous solution, in many cases, a long processing time as from several hours to several tens hours is required though it varies depending upon the desired processing shape or temperature conditions upon which the treatment is carried out, or the like.
For the purpose of shortening this processing time even a little, there is developed a chemical liquid exhibiting a high etching rate. For example, Patent Document 1 discloses a technology in which an aqueous solution obtained by adding a hydroxylamine in TMAH is used as an etching liquid. Also, Patent Document 2 discloses a technology in which an aqueous solution obtained by adding a specified compound such as iron, iron(III)chloride, iron(II)hydroxide, etc. in TMAH is used as an etching liquid and discloses that so far as a degree of the effect for increasing an etching rate is concerned, a combination of joint use of iron and hydroxylamine is especially suitable. Also, Patent Document 3 discloses a technology in which an aqueous solution obtained by adding a hydroxylamine in KOH is used as an etching liquid.    [Patent Document 1] JP-A-2006-054363    [Patent Document 2] JP-A-2006-186329    [Patent Document 3] JP-A-2006-351813    [Non-Patent Document 1] Sato, “Silicon Etching Technologies”, Surface Technology, in Journal of the Surface Finishing Society of Japan, Aug. 1, 2000, Vol. 51, No. 8, 2000, pages 754 to 759    [Non-Patent Document 2] Esashi, “2003 MICROMACHINE/MEMS Technology Outlook”, Electronic Journal Co., Ltd., Jul. 25, 2003, pages 109 to 114